Alkali-metal/chalcogen battery cells, such as sodium/sulfur cells, in which the electrolyte/separator/membrane takes the form of a large number of hollow fibers (capillary tubules) are now well known. See, for example, U.S. Pat. Nos. 3,476,062; 3,672,995; 3,679,480; 3,703,412; 3,749,603; 3,765,944; 3,791,868; 3,829,331; 3,917,490 and 4,050,915.
In a typical such sodium/sulfur cell, a body of molten sodium (the anolyte, on discharge) is disposed above an electronically non-conductive tubesheet and extends into (and fills) the tubules, which have their open end at the upper tubesheet surface. The tubules depend from the lower surface of the tubesheet and are immersed in a body of molten catholyte--a solution and/or mixture of sodium polysulfide in sulfur. When the cell is discharging through an external working circuit, elemental sodium gives up electrons to an anodic electron collector and forms Na.sup.+ ions which pass through the tubule walls into the catholyte. Sulfur in the catholyte takes up electrons at the cathodic current collector to form one polysulfide (S.sub.x.sup.2-) ion for every two Na.sup.+ ions formed.
It is highly important, in handling tubesheet/fiber assemblies, to avoid breaking off even one of the fibers, which are generally so small and so composed as to be fragile. It was conceived that the breakage tendency could be reduced if the flexure of the fiber, where it emerges from the bottom of the tubesheet, could be made less abrupt, i.e., could be distributed over a portion of the fiber length, rather than concentrated at the point of emergence. An ideal--but difficult to attain--improvement would be to have each fiber emerge from its own "well" (in the lower tubesheet surface) having the general shape of a trumpet bell. However, a further consideration is that a fluxing action of the tubesheet material on the fiber walls during densification (fusing) of the tubesheet apparently augments the tendency of the pendant fiber portions to break off, and this would not be eliminated by the latter improvement.
A modification which would appear to approach the ideal shape of the lower tubesheet surface and also to move the zone of fiber weakening up into the tubesheet body (where it is of much less concern) is to minimize the degree of contact between the fiber walls and a bottom portion (layer) of the tubesheet material. This might be done by utilizing a more open internal tubesheet structure but maintaining the essential impermeability of the tubesheet and sealingly engaging it with the fibers then become problems.